This invention relates to apparatus for and a method of effecting call handover in a radio telephone system in which a mobile radio moves with respect to cables which are leaky or lossy cables radiating radio fields detected by the mobile radio.
Call handover occurs when the mobile radio transfers communication from one cable to another cable. An example of such a known system is in an underground railway. Each railway station is equipped with a radio base station serving that particular railway station. Also, each base station serves an adjacent length of tunnel bore by means of a cable which extends from the base station along the tunnel bore both towards the preceding station and towards the succeeding station, in relation to the direction of travel of trains through the tunnel bore. The train carries the mobile radio, the front of the train being fitted with an antenna which makes radio communication with the cable extending along the tunnel bore. In proceeding from one railway station to the next railway station, the mobile radio must therefore transfer its communication (ie effect call handover) from one cable to another cable, this taking place approximately half-way between the railway stations where one cable ends and the next begins.
In known systems, call handover relies upon the detection by the mobile radio of the reduction of radio field as the mobile radio reaches the end of the cable. This triggers the mobile radio to search for a new channel available at the base station located at the railway station towards which the train is travelling. During call handover, an existing call will be uninterrupted only if the mobile radio can continue to communicate with the cable about to terminate. Call handover typically takes a quarter of a second or slightly more, which at 110 km per hour corresponds to a distance travelled of about 30 meters. The length of train tends to shield the antenna after the front of the train has moved 10 meters or more from the end of the cable, so there will be a partial loss of call. The invention aims to provide an apparatus and a method which overcomes this problem.